1. Field of the Invention
The invention relates to remotely adjustable instrumentation, such as aircraft flight instruments and more particularly to apparatus for remotely adjusting positionable reference indicia of such flight instruments.
2. Description of the Prior Art
The positioning of reference cursors, pointers, etc. of many types of flight instruments from a remote location is a desirable feature in modern day aircraft. Pilot workload is reduced or made more convenient by permitting him to initiate control and adjustments of the various indicia from a central control location within an easy reach. Typical of the flight instruments in which remote adjustment is desirable is a horizontal situation indicator (HSI), which integrates many display parameters associated with the lateral plane of flight and normally includes manually settable reference indicia or cursors such as a heading select cursor and a course select cursor, which may have to be adjusted many times during a typical flight.
In the prior art, the heading select cursor and the course select pointer have been adjusted remotely by a conventional servomotor controlled by a back-to-back knob/synchro, servo synchro arrangement. More particularly, a rotatable knob is mechanically coupled, by means of a fixed gear ratio, to the rotor of a remote transmitter synchro. The stator of the synchro transmits rotor positional data to the stator winding of a receiving synchro disposed at the flight instrument whereupon the appropriate voltage is induced in the rotor winding thereof. The rotor output is applied to a servomotor which adjusts the heading cursor or select pointer accordingly and repositions the receiver rotor until its signal goes to zero.
Utilizing such servo systems to remotely adjusting flight instruments, has several disadvantages. Firstly, a servo mechanism is costly with its gearing, motor, amplifier and synchro along with the remote controller synchro and gearing. Secondly, the heading knobs on the indicator itself are rendered useless since the servo will act to return heading cursor to the former position as dictated by the remote knob unless complex mechanical differential clutch means are incorporated further increasing the over-all instrument cost. Thirdly, the alignment and calibration of a servo system must be performed periodically due to wear on the gears, etc. thereby increasing maintenance costs and total cost of ownership.